1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
There has been proposed an image forming apparatus of a type, in which an electrostatic latent image is formed on the surface of a photosensitive member. The electrostatic latent image is then developed by toner applied to the photosensitive member. The toner image is then transferred to a recording medium.
As shown in FIG. 1A, this type of image forming apparatus 300 includes: a photosensitive drum 320, a charging unit 360, a developing roller 330, a supply roller 340, a layer-thickness regulating blade 380, and a toner tank 350. The surface of the photosensitive drum 320 is uniformly charged by the charging unit 360. An electrostatic latent image is formed on the surface of the photosensitive drum 320. The supply roller 340 supplies toner from the toner tank 350 to the developing roller 330. More specifically, the developing roller 330 receives toner from the supply roller 340 at a supply point A. The layer-thickness regulating blade 380 removes the excess amount of toner from the developing roller 330. The developing roller 330 supplies a proper amount of toner to the surface of the photosensitive drum 320.
In this way, when toner is supplied to the developing roller 330 from the toner tank 350 by the supply roller 340, excess toner is removed by the layer-thickness regulating blade 380 so that the thickness of the layer of toner on the developing roller 330 becomes uniform. The uniform thickness of toner layer is supplied from the developing roller 330 to the photosensitive drum 320. The toner adheres to the area of the photosensitive drum 320 where the electrostatic latent image is formed. As a result, the electrostatic latent image is developed. A sheet of paper is transported along a transport path 370. The developed toner image is transferred onto the sheet that is being transported along the transport path 370, as a result of which an image is formed on the sheet.
In this example shown in FIG. 1A, the image forming apparatus 300 employs an “opposite-direction” developing method (“against” developing method) for developing the electrostatic latent image. According to the opposite-direction developing method, the photosensitive drum 320 and the developing roller 330 both rotate in the same direction (clockwise direction, in this example) as indicated by the arrows in FIG. 1A. Therefore, the surface of the photosensitive drum 320 and the surface of the developing roller 330 are moving in the opposite directions at the position where they are in contact to develop the electrostatic latent image.
The layer-thickness regulating blade 380 is disposed above the supply point A. The transport path 370 is located above the photosensitive drum 320. Accordingly, the route from a paper supply operation to an image-fixing operation can be arranged in one line. Many kinds of sheets of paper can be used for printing. The image forming apparatus 300 can be made small in size.
The image forming apparatus 300 can be modified to employ a “same-direction” developing method (“with” developing method) as shown in FIG. 1B. According to the same-direction developing method, the photosensitive drum 320 and the developing roller 330 rotate in opposite directions. In this example, the photosensitive drum 320 rotates in the clockwise direction, while the developing roller 330 rotates in the counterclockwise direction as indicated by the arrows in FIG. 1B. Therefore, the surface of the photosensitive drum 320 and the surface of the developing roller 330 are moving in the same direction at the position where they are in contact with each other to develop the electrostatic latent image.
In this case, the layer-thickness regulating blade 380 is disposed below the supply point A. The transport path 370 is located above the photosensitive drum 320. Accordingly, the route from the paper supply operation to the image-fixing operation can be arranged in one line. Many kinds of sheets of paper can be used for printing. The image forming apparatus 300 can be made small in size.
As described above, the position of the layer-thickness regulating blade 380 will vary depending on the employed method of developing the electrostatic latent image.
As shown in FIG. 1B, in the same-direction developing method, the layer-thickness regulating blade 380 is disposed below the supply point A. Accordingly, the excess toner will fall downward and accumulate in the casing (not shown in the drawings) of the image forming apparatus 300. In order to re-use and recycle the toner, it is necessary to return the toner to such a position that the toner can be supplied to the developing roller 330.
Contrarily, according to the opposite-direction developing method, as shown in FIG. 1A, the layer-thickness regulating blade 380 is disposed above the supply point A. This has the advantage that the excess toner will drop back to the supply point A and will be supplied again to the developing roller 330. For this reason, in recent years, the opposite-direction developing method is employed in many image forming apparatuses.
The toner used in the image forming apparatus 300 mainly includes: pigment, resin, and wax. The pigment can be carbon black, for example. The resin is for fixing the pigment to the sheet. The wax is a supplementary fixing agent.
There are mainly two types of toner: pulverized toner, and polymer toner. Pulverized toner is prepared by mixing the pigment, resin and wax mechanically and then by pulverizing the mixture. Polymer toner is prepared by coating the pigment and wax with resin by polymerization to form toner particles of approximate sphere shapes.
The fluidity of pulverized toner is lower than that of polymer toner. Therefore, when pulverized toner is used, fogging can easily occur, and the printing quality is reduced considerably. More specifically, because the fluidity of pulverized toner is not so good as that of polymer toner, the opportunities for pulverized toner to come in contact with the developing roller or blade or the like is lower than that for polymer toner. Hence, toner charged with insufficient charge amount or toner charged to an opposite polarity occur and erroneously develops non-image forming area of the photosensitive drum, onto which toner should not attach. This results in fogging of a resultant image.
On the other hand, the fluidity of polymer toner is high, and therefore this type of fogging does not occur. Higher quality printing is possible in comparison to pulverized toner. For these reasons, it is desirable to employ the opposite-direction developing method and to use polymer toner.